How are scales formed in boilers?
The limescale that forms in boilers (and electric kettles) is formed by the thermal decomposition of the calcium (and magnesium) hydrogencarbonates present in temporarily hard water. e.g. Ca(HCO3)2(aq) --- CaCO(s) + H2O(l) + CO2(g) The solid calcium (or magnesium) carbonate (the limescale) builds up on the hot surfaces.
There are two kinds of scale. There are scales formed from minerals that come in with the boiler feed water, largely hardness scales from calcium and magnesium. They tend to form in the hot zones of the boiler because they get less soluble as temperature increases. That is why high purity water is normally used. Other scale is formed from iron and copper corrosion products from piping and heat exchangers in the feed water circuits. Again, they precipitate out in the hottest parts of the boiler tubing.
